1. Technical Field
This invention relates to a voltage follower circuit in a linear integrated circuit adapted for video signal processing and other electrical equipment.
2. Description of the Prior Art
In general, a voltage follower circuit is used as an impedance transfer circuit or a distributor. FIG. 1 shows one example of a voltage follower circuit. In FIG. 1, transistors Q1 and Q2 form a differential amplifier, and the emitters of these transistors are connected to a constant current source 10 which includes a transistor Q3 and a resistor R1. The collector of transistor Q1 is connected to voltage source Vcc. The collector of transistor Q2 is connected to a voltage source Vcc through constant current source 12 which includes transistor Q4 and resistor R2. The base of transistor Q2 is grounded through constant current source 14 which includes transistor Q5 and resistor R3. The base of transistor Q2 is also connected to voltage source Vcc through transistor Q6. The base of transistor Q6 is connected to the collector of transistor Q2. The base of transistor Q1 is connected to voltage source VB1 through input signal source VIN. The bases of transistors Q3 and Q5 are connected to voltage source VB2. The base of transistor Q4 is connected to voltage source VB3.
In the above circuit, the voltage sources Vcc, VB1, VB2 and VB3 equal 5 V, 2.0 V, 0.9 V and 4.1 V, respectively, and VIN is a 2 V peak-to-peak signal. Accordingly, the voltage of an input signal of 100% white level would be 4.0 V, the sum of VIN and VB1 (see FIG. 2A). If the voltage follower circuit operated normally, the output of the base of transistor Q2 would be a video signal of 2.0 V at sync-tip level and 4.0 V at 100% white level. However the voltage at the collector of transistor Q2 is 2.7 V at sync-tip level and 4.7 V at 100% white level (see FIG. 2B showing the output of the base of transistor Q2), because the voltage at the collector of transistor Q2 equals the voltage at the base plus 0.7 V of the base-emitter voltage of transistor Q6. Thus, transistor Q4 saturates and the voltage follower circuit fails to operate normally. In other words, the dynamic range of transistor Q2 is small, so that an input video signal is not output normally.
The clamp voltage of the input signal applied to the base of transistor Q1 may be reduced by about 0.3 V. As a result of the reduction, the dynamic range of transistor Q2 will become greater, but it is still inadequate to prevent the above-described saturation problem from occurring.